Sheets or plates, et cetera



-atented Apr. 24, 1945 SHEETS R PLATES, ET CETERA James Henry Rooney, Ronald Stanley Locke, and Philip Richard Hawtin, Spondon, near Derby, England, assignors to British Celanese Limited, London, England, a company of Great Britain No Drawing. Application June 5, 1942, Serial No. 445,982. In Great Britain June 6, 1941 9 Claims. (Cl. 18-=57) leaves knife lines on the surface which have to be removed by a polishing process. Residual volatile solvent has to be removedby a prolonged period of seasoning, and after the seasoning the sheets have to be straightened. The process is a lengthy one and demands the use of expensive presses for forming the blocks, polishing and straightening the sheets. U. S. Patent application of A. J. Daly, P. R. Hawtin and B. Shaw S. N0.

346,056, filed July 17, 1940, which has matured as Patent No. 2,332,559, describes a simpler process presenting many advantages over the process described above. According to the process of the patent referred to, sheets which may be of considerable thickness are formed by casting on to v a flat polished surface of a glass plate a solution of cellulose acetate or other film-forming polymer in a volatile solvent, removing a sumcient proportion of solvent by evaporation to allow of the sheet being stripped from the plate, stripping said sheet and removing substantially the whole of the remaining volatile solvent from the stripped sheet. Thick sheets can be built up from a plurality of layers of film-forming solution, the bulk of the volatile solvent being removed from each layer before deposition of the next layer.

Dlficulty is experience in stripping thick sheets made by the patented process unless the amount of residual solvent in the sheet is controlledbetween narrow limits before stripping, and even when the solvent content of the sheet is so controlled, great care is necessary in stripping thick sheets if damage to the sheet and to the casting surface is to be avoided.

It is the principal object of the present invention to provide a process whereby even the thickest sheets can be formed by casting and removed without difficulty from the supporting surface, which may, for example, be of glass, as in U. S. Patent No. 2,332,559 or of metal or other suitable material capable of taking a high polish.

We have found that by laying a preformed sheet of the film-forming polymer on the smooth surface of a rigid support e. g.. a polished sheet of glass, thinly and evenly coated with a lubricant or other composition adapted to prevent adhesion of the preformed sheet to the surface, and casting further layers of cellulose acetate on to the preformed sheet, a substantially homogeneous sheet of any desired thickness, incorporating the preformed sheet, can be built up and readily stripped from the casting surface. The preformed sheet must be capable of lying flat on the surface of the support or a similar support, and of providing a flat surface, when in position, for the reception of the fresh layers of film-forming substance. It may, for example, be a foil cast on a film-forming band, but is preferably a sheet cast on the surface of the support, or 'a similar support, during a previous operation. It must be suiliciently thin to be stripped from that surface without dimculty.

The following example illustrates the invention:

Example On the flat highly polished surface of a glass plate a layer of cellulose acetate, containing about 30 to 40% of its weight of plasticiser, and about one hundredth of an inch thick, is formed by depositing a layer of dope containing celluloseacetate and the plasticiser dissolved in acetone, and exposing the layer of dope to a'warm atmosphere until the solvent content is reduced to about 10 to 15%. A further layer of dope is deposited on the first and partly dried as before and this process is repeated until a sheet about 0.04" thick has been built up. This sheet is stripped from the glass by traction applied at one corner to roll the sheet back on itself and constitutes the preformed sheet which serves as a foundation for v the final thick sheet. The glass plate or a similar glass plate is now provided with walls about 0.02" high terminating in fiat outwardly-projecting flanges parallel to the casting surface of the glass plate. The space formed by these walls and the surface of the plate is filled with a mineral oil. The preformed sheet is then laid fiat on the layer of oil and adhesively attached along its edges to the flanges on the walls. Twelve further layers of the dope are then deposited on the sheet, each layer being dried to about the same extent as the first layer, i. e. to a solvent content of about 10- 15%, before deposition of the next layer. In this way a homogeneous sheet about.0.l6" in thickness, and incorporating the preformed sheet, is built up. This sheet can readily be stripped from the glass plate, after removing the side walls which retain the oil. The sheet is then held flat while residual volatile solvent is removed, for example by exposure to a current of warm air, after which the sheet may be finished as desired.

Care'is necessary to avoid the formation of bubbles in the sheets. To this end dust must be rigidly excluded from the film-forming apparatus, the dope must be carefully filtered and free from dissolved gases, and the rate at which solvent is evaporated from the layers of dope must-not be unduly high. Dissolved gases may be removed from the dope by heating it under reduced pressure before use. Thus, for instance the dope may be made up to contain initially a slight excess of volatile solvent, for instance more than is required in casting the layers, and this excess solvent may be boiled off under reduced pressure before the casting operation.

The dope may contain any suitable plasticiser for the cellulose acetate or other film-formin substances. Among plasticisers for cellulose acetate mention may be made of triacetin, dimethyl phthaiate, dimethoxy, ethyl phthalate, dibutyl phthalate, di(ethoxy-ethyl)-phthalate, methyl (methoxy-ethyl)-phthalate, ethyl (ethoxy-ethyl) phthalate, and diethyland dibutyl tartrates. Plasticlsers which are without substantial ailinity for the cellulose acetate but which are useful with a view to increasing the fire resistance of the sheet material, e. g. tricresyl phosphate, triphenyl phosphate and trichloro-ethyl phosphate, maybe present in the composition, preferably in admixture with plasticisers having a high affinity for cellulose acetate, e. g. triacetin. A very suitable mixture of plasticiser for cellulose acetate is: three parts by weight of dimethyl phthalate to one of tricresyl phosphate. The dope may also contain volatile solvents for cellulose acetate other than acetone, e. g. dioxane or methylene ethylene ether. High boiling solvents e. g. diacetone alcohol and ethyl lactate may also be present in addition to the volatile solvent.

In a similar way sheets of other acetone soluble cellulose derivatives especially other lower fatty acid esters of cellulose containing acetyl groups, e. g. cellulose acetate-propionate or cellulose acetate-butyrate, Or of other non-elastic artificial film-forming polymers, especially other organic derivatives of cellulose, can be formed.

In carrying out the process of the invention methods and apparatus such as are described in US. application of H. Dreyfus, No, 390';? 55, filed April 25, 1941, may be employed togachieve mass production of the sheets. Thus, preformed sheet can be built up by flowing the dope continuously on to a series of casting plates, there being relative motion between the series of plates volatile solvent can be removed before stripping this sheet, provided that the sheet is not too thick. Instead of a mineral oil other liquid lubricants may be employed, for example castor oil and olive oil. It is an advantage of the process of the invention that liquid lubricants can be employed between the surface of the glass and of the sheet to be stripped. Such lubricants not only greatly facilitate stripping but also by virtue of their mobility enable temporary unevennesses in the under surface of the sheet to be rectified by the forces called into play by evaporation of the solvent. Attempts to build up a thick sheet directly on to the surface of the liquid lubricant without the use of a preformed sheet, have not been successful in that they have resulted in a sheet of uneven thicknes or having a wavy or rippled under surface.

Solid or semi-solid lubricants can also be employed; for example, the glass can be provided with a suitable surface by pouring on to it a soap, an ester of a soap forming acid, or other soap-like substance molten or in aqueous solution, and setting the layer so formed by cooling on evaporation. The preformed sheet of cellulose is then applied to the layer of soap or the like, under pressure, preferably applied by a heated platten so as to polish the surface of the soap layer. The thick sheet is then built up with the preformed sheet as a basis as described above. Similarly a layer of petroleum jelly can be used. Another alternative is to coat the support before applying the preformed sheet, with a non-elastic film forming polymer which is insoluble in the solvent employed in the dope from which the sheets are formed, as described in U. S. application of H. Dreyfus, S: No. 455,503, filed August 20, 1942. For instance when casting sheets from acetone solutions of organic derivatives of cellugo lose the support may be surfaced with a water and the flowing point, and, after stripping each plate, lubricating the'surface, and replacing the preformed sheet, the final sheet may be builtup by continuously flowing the dope on to the series of preformed sheets, each carried on the lubricated surface'of its plate, there being relative movement between the plates and the flowing point as before. Each layer of dope is dried to the desired extent before the deposition upon it of a further layer of dope.

The preformed sheet need not be as thick as 0.04" nor need it be formed from several layers of dope. It may be less than 0.02" in thickness, e. g. 0.01" and be formed from a single layer of dope. It must be sufficiently thin to be flaton the film-casting. surface at least during the final stages of building up the-thick sheet. Instead of stripping the preformed sheet from the glass after reduction of the volatile solvent content to about It to 15%, submfltially the whole of the soluble saponified olyvinyl acetate before applying the preformed sheet. Solid substances with lubricating properties, e. g. graphite, are not in general satisfactory.

Instead of cellulose acetate other thermoplasticorganic polymer capable of forming strong self-supporting films may be employed, for example cellulose proplonate, cellulose butyrate, cellulose acetate-propionate, cellulose acetate-- butyrate, cellulose acetate-nitrate, ethyl cellulose, benzyl cellulose, ethyl cellulose acetate and other esters, including mixed esters, ethers and ester-ethers of cellulose. The film-forming polymer need not be a cellulose derivative. It may, for example, be a polymerised unsaturated substance such as polyvinyl alcohol, polyvinyl acetate, polyvinyl chloride, polyvinyl chloracetate, a co-polymer of two or more of such unsaturated substances, a polymerised unsaturated compound derived from acrylic-acid, e. g. polymethyl methacrylate, a polymerised unsaturated hydrocarbon, e. g. a polystyrene synthetic resin, or any other organic film-forming polymer from which stron self-supporting films, foils or the like can be made by the evaporative method. I I

The film-forming polymer must be of high molecular weight, e. g. at least 3,000 or 4,000 and preferably at least 5,000 measured by the Staudinger viscosity method. At the same time it must be of such a nature that a volatile solvent can be found for it, and be capableof formcludes the production of coloured and opaque sheet materials. Such materials maybe made by incorporating dyes, pigments and other effect materials in the dope.

Having described our invention what we desire to secure by Letters Patent is:

1. PlOCeSs for the production of thick sheets of an -artificial non-elastic film forming polymer which comprises preforming a sheet by depositing a solution of said polymer in a volatile solvent on the smooth surface of a rigid support and tatein acetone on the smooth surface of a rigid support, and evaporating said acetone, stripping the preformed sheet from the support, thinly and evenly coating the support with a soap, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by deposition of said solution thereon followed by evaporation of acetone, stripping the remaining acetone,

evaporating said solvent, stripping the preformed sheet from the support, thinly and evenly coating the support with a material adapted to prevent adhesion between the polymer and the surface, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by deposition of' said solution thereon followed by evaporation of solvent, stripping the resulting sheet incorporating the preformed sheet from the surface, and evaporating substantially all remaining volatile solvent.

2. Process for the production of thick sheets of a thermoplastic cellulosic compound, which comprises preforming a sheet by depositing a solution of said thermoplastic cellulosic compound ina volatile solvent on the smooth surface of a rigid support, and evaporating said solvent, stripping the preformed sheet from the support, thinly and evenly coating the support with a material adapted to prevent adhesion between said thermoplastic cellulosic compound and the surface, laying the preformed sheet upon the coated surface, building up the desired sheetupon the preformed sheet by deposition of said solution ther on followed by evaporation of solvent, 'stripp' g the resulting sheet incorporating th a preformed sheet from the surface and evaporating substantially all remaining volatile solvent.

3. Process for the production of thick sheets of cellulose acetate, which comprises preforming a sheet by depositing a solution of cellulose acetate in a volatile solvent on the smooth surface of a rigid support, and evaporating said solvent, stripping the preformed sheet fromthe support,

- thinly and evenly coating the support with a material adapted to prevent adhesion between said cellulose acetate and the surface, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by deposition of said solution thereon followed by evaporation of solvent, stripping the resulting sheet incorporating the preformed sheet from the 6. Process for the production of thick sheets of cellulose acetate, which comprises preforming a sheet by depositing a solution of cellulose acetate in acetone on the smooth surface of a rigid support, and evaporating said acetone, stripping the preformed sheet from the support, thinly and evenly coatingthe support with an ester of a soap-forming acid, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by deposition of said solution thereon followed by evaporation of acetone, stripping the resulting sheet incorporating the' preformed sheet from the surface and evaporating substantially all remaining acetone.

7. Process for the production of thick sheets of cellulose acetate, which comprises preforming a sheet by depositing a solution of cellulose acetate in acetone on the. smooth surface of a glass plate, and evaporating said "acetone, stripping the preformed sheet from the glass plate, thinly and evenly coating the glass plate with an oil, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by deposition of said solution ther e g followed by evaporation of acetone, stripping the resulting sheet incorporating the preformed sheet from the surface and evaporating substantially all remaining acetone. 3

8. Process for the production of thick sheets of cellulose acetone, which comprises preforming a sheet by depositing a solution of cellulose acethe preformed sheet from the glass plate, thinly surface and evaporating substantially all remaining volatile solvent.

4. Process for the production-of thick sheets of cellulose acetate, which compri'sespreforming a sheet by depositing a solution of cellulose acetate in acetone on the smooth surface of a rigid support, and evaporating said acetone, strip ping the preformed sheet from the support, thinly and evenly coating the support with an oil, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by depofltion of said solution thereon followed by evaporation of acetone, stripping the resulting sheet incorporating the preand evenly coating the glass plate with a soap, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by deposition of said solution thereon followed by evaporation of acetone stripping the resulting sheet incorporating the preformed sheet from the surface and evaporating substantially all remaining acetone.

9. Process for the production of thick sheets of cellulose acetate, which comprises preforming a sheet by depositing a solution of cellulose acetate in acetone on the smooth surface of a glass plate, and evaporating said acetone, stripping the preformed sheet from the glass plate, thinly and evenly coating the'glass plate with an ester of a soap forming acid, laying the preformed sheet upon the coated surface, building up the desired sheet upon the preformed sheet by deposition of said solution thereon followed by evaporation of acetone, stripp n the resulting sheet incorporating the preformed sheet from the surface and evaporating substantially all remaining acetone.

JAMES HENRY RODNEY. RONALD STANLEY LOCKE.

RICHARD 

